Air filters with particularly high collection efficiency are generally referred to as High Efficiency Particulate Air (HEPA) filters. HEPA filters are employed extensively in the microelectronics field, for example, in clean rooms and in the pharmaceutical industry. They are also used in hospitals, in food and cosmetic production facilities, as well as in residential settings, e.g., in air purifiers and vacuum cleaners.
HEPA filters are generally fiber-based and are made up of an entanglement of thin fibers that usually are less than one micron in diameter. With fiber-based materials, particles are collected by one or more classical mechanisms, such as diffusion, interception or inertial impaction. Two important performance-related parameters associated with these filters are pressure drop and collection efficiency. Collection efficiency (E) is related to penetration (P) by the formula: P=1−E. These performance-related parameters depend on several factors, such as: filter structure, e.g., packing density or fiber diameter; operating conditions, e.g., filter velocity, temperature; properties of the materials being filtered, e.g., density, mean particle size, particle size distribution, solid or liquid; or filter loading.
SEM studies indicate that in the case of solid particles filter loading of HEPA fiber filters initially takes place in the depth of the filter with the formation of chain-like agglomerates called dendrites. During the early stages of the filtration process, and at constant face velocity, the pressure drop across the filter generally rises linearly with the amount of mass or particles collected. However, as the dendrites begin to fill the spaces between the fibers, a filter cake of increasing thickness begins to form at the upstream surface of the filter and the slope of the pressure drop with increasing loading rises sharply, indicating that the filter is being clogged.
Shown in FIG. 1 is a schematic diagram of gas flow through a conventional HEPA filter, illustrating bulk region 3 and formation of cake 5 at the surface of filter 7 resulting in clogging of the filter. As solid particles deposit over the layer of fibers, the gaps are filled and the collection efficiency improves. However, the resistance of the filter increases, limiting the operative life of the filter. Thus “surface” or “cake” filtration has good collection efficiency, low capacity and is accompanied by a rapid increase in pressure drop as the surface gets clogged.
In the case of liquid particles or mists, particles are first deposited as droplets around the fibers and the pressure drop rises slowly with mass collected per unit of filter area. At a certain point during filtration, however, a sharp exponential rise in pressure drop is observed. This behavior generally is attributed to the presence of a liquid film covering the filter surface. It is believed that droplets deposited progressively grow and join together to form bridges at the intersection of several fibers. At the point of clogging, all, or substantially all, of the interstices of the first layer of fibers are filled, forming a film covering the filter surface. It is noted that clogging occurs at a much higher loading level for liquid particles, e.g., mists, than for solid particles.
Generally, particles that are the most difficult to filter are submicron in size and are referred to as particles having the “most penetrating particle size” or “MPPS”. When clean, HEPA fiber-based filters provide excellent filtration efficiency and a low pressure drop for both solid and liquid MPPS particles and filtration occurs throughout the depth of the filter. This type of filtration generally is known as “deep bed filtration”. However, as soon as the upstream surface becomes heavily clogged, filtration only occurs at the filter's surface, a phenomenon known as “cake filtration”, leading to a sharp rise in pressure drop. Based on this sharp rise in pressure drop, filtration performance becomes unacceptable and the filter needs to be cleaned or replaced. In typical fiber-based HEPA filters, this degradation in performance occurs at a loading of about 1 to 10 g/m2 of filter area.
MPPS oil droplets tend to coalesce and penetrate voids in conventional HEPA filters. Since gas velocity through the filter increases due to clogging, oil that has saturated the voids of the fiber-based filter could be released in the form of liquid drops or as a liquid film downstream of the filter. Whereas, due to cake formation over the surface of the filter, collection efficiency of HEPA filter can actually increase when challenged with solid particles, the collection efficiency for oil droplets tends to decrease with time, due to a reduction in fiber surface available for capturing incoming droplets, as the fibers become coated with liquid.
Other techniques have been tried in the removal of oil droplets from air and they include demisters or woven metallic wire as means for collecting droplets by coalescence, electrostatic filters, centrifugal collectors and scrubbing. As with fiber-based HEPA filters these approaches do not appear to be very effective in handling submicron size droplets.